The optic nerves are the second pair of cranial nerves, coming directly from the brain and distributed to the eye retina. If the optic nerves are traced and followed in their course from the back of the retina to the brain, it is noticed that they come out through openings in the bony eye sockets, and as they pass backward the two nerves come together and their fibers mix.
Then some of the fibers cross each other like the letter X, so that those coming from the inner side of the left retina pass to the right side and those coming from the inner side of the right retina pass to the left side of the brain, and thus each side of the brain gets nerve fibers from both eyes.
After the optic nerves cross each other, each nerve, now called the "optic tract," passes along to the base of the brain entering the part of the brain called the "optic thalamus."
In the optic thalamus the optic nerve fibers make connection with the nerve cell substations (ganglia) and other structures, and then the optic fibers with connecting fibers from these ganglia pass backward to end in the extreme back part of the brain, the "cortex cells" of the occipital brain lobe.
That is the reason why eyestrain and eye pain cause a headache in the back of the head, because that is where the optic nerves begin.
When an image is thrown on the retina , the special cells (rods and cones ) which connect with the optic nerve transmit the sensation through the nerve to the optic thalami substations, where the sensation of sight is acknowledged.
And from there the operators on the main lines relay the sensations to the brain cells, where the sight sensations are deciphered and translated into a recognized and definite image.
It is evident that, because nerve fibers from the inner side of the left eye cross to the right, joint the fibers from the outer side of the right eye and both enter the right side of the brain, therefore an injury, blood clot or tumor in that part of the brain will cause blindness in both the inner side of the left eye and the outer side right eye, a corresponding effect will result if the left side of the brain becomes affected.
Blindness may result when any part of the optic nerve is pressed upon by a blood clot or tumor. The optic nerve, as it passes to the base of the brain, is very close to the pituitary gland, and this gland is sometimes affected by growths which will press on the optic nerve and therefore produce atrophy and blindness